1. Field of the Invention
The present invention relates to a polishing apparatus and method, and more particularly to a polishing apparatus and method for polishing an object to be polished (substrate) such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
In recent years, high integration and high density in semiconductor device demands smaller and smaller wiring patterns or interconnections and also more and more interconnection layers. Multilayer interconnections in smaller circuits result in greater steps which reflect surface irregularities on lower interconnection layers. An increase in the number of interconnection layers makes film coating performance (step coverage) poor over stepped configurations of thin films. Therefore, better multi layer interconnections need to have the improved step coverage and proper surface planarization. Further, since the depth of focus of a photolithographic optical system is smaller with miniaturization of a photolithographic process, a surface of the semiconductor device needs to be planarized such that irregular steps on the surface of the semiconductor device will fall within the depth of focus.
Thus, in a manufacturing process of a semiconductor device, it increasingly becomes important to planarize a surface of the semiconductor device. One of the most important planarizing technologies is chemical mechanical polishing (CMP). Thus, there has been employed a chemical mechanical polishing apparatus for planarizing a surface of a semiconductor wafer. In the chemical mechanical polishing apparatus, while a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface such as a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface, so that the substrate is polished.
This type of polishing apparatus includes a polishing table having a polishing surface formed by a polishing pad, and a substrate holding device, which is referred to as a top ring or a polishing head, for holding a substrate such as a semiconductor wafer. When a semiconductor wafer is polished with such a polishing apparatus, the semiconductor wafer is held and pressed against the polishing surface under a predetermined pressure by the substrate holding device. At this time, the polishing table and the substrate holding device are moved relative to each other to bring the semiconductor wafer into sliding contact with the polishing surface, so that the surface of the semiconductor wafer is polished to a flat mirror finish.
In such polishing apparatus, if the relative pressing force applied between the semiconductor wafer, being polished, and the polishing surface of the polishing pad is not uniform over the entire surface of the semiconductor wafer, then the surface of the semiconductor wafer is polished insufficiently or excessively in different regions thereof, which depends on the pressing force applied thereto. It has been customary to uniformize the pressing force applied to the semiconductor wafer by providing a pressure chamber formed by an elastic membrane at the lower portion of the substrate holding device and supplying the pressure chamber with a fluid such as compressed air to press the semiconductor wafer against the polishing surface of the polishing pad under a fluid pressure through the elastic membrane.
On the other hand, a thin film formed on a surface of a semiconductor wafer as an object to be polished has different thicknesses depending on radial locations of the semiconductor wafer due to characteristics of a film-forming method or apparatus. Specifically, the thin film formed on the surface of the semiconductor wafer has an initial film thickness distribution in its radial direction. Therefore, as described above, in the substrate holding device for pressing the entire surface of the semiconductor wafer uniformly and polishing the semiconductor wafer, the semiconductor wafer is polished uniformly over the entire surface thereof, and thus the above initial film thickness distribution on the surface of the semiconductor wafer cannot be corrected. Therefore, as disclosed in Japanese laid-open patent publication No. 2006-128582, there has been proposed a polishing apparatus in which a plurality of pressure chambers are formed by an elastic membrane within a surface of the semiconductor wafer, and pressures of a pressurized fluid such as compressed air to be supplied to the respective pressure chambers are independently controlled to control pressures applied to the semiconductor wafer at the respective areas on the semiconductor wafer. Therefore, in such a polishing apparatus, a pressing force for pressing the semiconductor wafer against the polishing surface is made larger at the area having a thick film than that at the area having a thin film, and the polishing rate of the area having the thick film is selectively enhanced to polish the semiconductor wafer flatly over the entire surface of the semiconductor wafer without relying on the film thickness distribution at the time of film-forming.
When a substrate such as a semiconductor wafer is polished by the polishing apparatus having the above structure, the substrate is pressed against the polishing surface of the polishing pad under a certain polishing pressure and is brought into sliding contact with the polishing surface. As a result, a temperature of contact surface of the substrate with the polishing pad, i.e., a polishing temperature is raised. As described above, it is important to control the polishing pressure for the purpose of improving polishing performance. However, it is also very important to measure and control the polishing temperature for the purpose of improving the polishing performance. Specifically, because the polishing pad is composed of a resin material such as foamed polyurethane, the polishing temperature changes rigidity of the polishing pad to have an influence on planarization characteristic of the substrate. Further, since the chemical mechanical polishing (CMP) is a polishing method which utilizes a chemical reaction between a polishing liquid (polishing slurry) and a surface, being polished, of the substrate, the polishing temperature has an influence on chemical characteristic of the polishing slurry. Further, the distribution of the polishing rate changes depending on the polishing temperature to deteriorate the yield rate or to lower the polishing rate, thus deteriorating productivity of the polishing apparatus. Further, if there is temperature distribution within the surface of the substrate, the polishing performance within the surface of the substrate becomes nonuniform.
Therefore, as disclosed in Japanese laid-open patent publication Nos. 2002-301660 and 2005-268566, a temperature of a substrate such a semiconductor wafer is measured during polishing. Further, as disclosed in Japanese laid-open patent publication No. 2006-332520, a temperature of a membrane for holding a semiconductor wafer is measured as a means for measuring a temperature of a portion near a surface, being polished, of the semiconductor wafer during polishing.